The present invention relates to a count discriminating fire detection system in which a predetermined number of pulses obtained from the output of a fire sensor are counted to issue an alarm, and more particularly to a system including counting means which is reset whenever a series of consecutive detection pulses less than the predetermined number ends.
A type of prior count discriminating fire detection system uses a capacitor which is charged to a predetermined voltage by applying the detection output or the pulses produced by a fire sensor to energize an alarm circuit. However, various disturbance signals or outputs which may be produced from a fire sensor when false alarm triggers arise such as smoking, the burning of small pieces of paper, steaming and so on also charge the capacitor. Therefore, this capacitor should be periodically discharged. Complex circuitry is required to periodically discharge the voltage stored in the capacitor thus employed.
Another type of such a system is disclosed in U.S. Pat. No. 3,842,409, which uses a shift registor and a capacitor in combination. A control circuit is connected between this capacitor (which is also connected to a data terminal of the shift registor) and an output terminal of the fire sensor to repetitively charge and discharge the capacitor, so that the shift registor may be reset by a clock pulse upon disappearance of the data signal at the data terminal in response to the discharge period of the capacitor. This system should be improved because of its complexity.